Functional eukaryotic messenger RNAs for several proteins have been shown to contain a poly(adenylic acid) or poly(A) sequence of 150-250 adenylate residues at their 3'-termini. Polyadenylation of RNA is presumed to be catalyzed post-transcriptionally by an enzyme called poly(A) polymerase which has been characterized in the nuclei. Mitochondrial mRNA also appears to contain a similar poly(A) sequence. An enzyme catalyzing the polyadenylation of mitochondrial mRNA was first identified in our laboratory (Jacob and Schindler, Biochem. Biophys. Res. Commun., 48, 126, 1972; and Science, 178, 639 1972). Subsequently, this enzyme was purified (Jacob et al., Biochim. Biophys. Acta, 361, 312, 1974; Rose et al., Biochemistry, 14, 1025, 1975). These studies demonstrated that a series of rapidly growing hepatomas contain only 50% as much mitochondrial enzyme/mg mitochondrial protein as the normal liver. Also, the endogenous primer associated with the extracted enzyme was not as efficient as the endogenous primer for normal liver. That broad objectives of this research project are (a) to compare the properties of mitochondrial poly(A) polymerase from a series of Morris hepatomas with varying growth rates and differentiation (the most highly differentiated hepatoma being closer to liver histologically and biochemically) with those of normal liver, (b) to elucidate any relationship of this enzyme to mitochondrial RNA polymerase, (c) to compare the properties of mitochondrial poly(A) polymerase with those of nuclear poly(A) polymerase, especially with respect to their primer specificities, (d) to purify and characterize the endogenous primers that are usually associated with the extracted enzyme and to elucidate its relationship to poly(A)-containing RNA from the mitochondria.